Recent research has shown a beneficial effect of microwave energy on the combustion processes of air-fuel mixtures. Microwave enhanced combustion (MEC) boosts combustion kinetics by accelerating flame electrons and creating radicals using microwave energy. MEC has demonstrated promising results in improving thermal efficiency and emissions for different engine platforms by improving flame speed, dilution tolerance, and combustion stability.
The MEC concept requires the combustion chamber to couple with the microwave energy field, leading to efficient energy transfer between the microwaves and the flame front as it travels across the chamber. This coupling is only achieved when the impedance of the microwave system matches the impedance of the combustion chamber.
As conventionally embodied, MEC transmits microwaves to the combustion flame only during the early combustion phase. After the first 3% to 5% of the combustion duration, around 90% of microwave energy is reflected to the transmitter before being emitted to flame. This large reflection limits MEC's ability to enhance combustion and causes device heating and potential damage.
The MEC reflection is caused by a radio frequency phenomenon known as impedance mismatch. To mitigate reflection, the impedance of the microwave transmitter should be matched to the impedance of the flame, which changes during combustion. In an internal combustion engine, an additional source of impedance variation comes from the changing volume of the combustion chamber due to the motion of the piston. Both sources of impedance variation can lead to an impedance mismatch.
Conventional MEC impedance matching devices are based on tuning screws or waveguide stubs, which are typically adjusted only before engine ignition. In addition, because the timing of the ignition event can change every engine cycle, and the optimal impedance match using conventional equipment can only occur at one fixed timing, it is difficult to ensure that an impedance mismatch will not occur in a running engine.